It is frequently desirable or necessary to measure small amounts of water in materials. For example it is frequently necessary to determine the humidity of a gas, the amount of water in an organic liquid or solvent such as petroleum, alcohol or an oil, the amount of water in a solid such as paper, pigment, fabric or coal or the water permeability of a material such as a packaging material.
In addition to determining the free water content of a material it is also useful to determine the capillary water content. Capillary water is water that is bound in small interstices of a crystalline material or is absorbed on material in a manner such that it does not merely evaporate from the material but requires greater energy to drive it off. It is also useful to determine the water of hydration or water of crystallization of a material. Water of hydration or water of crystallization is water that is associated with molecules of salts whereby some chemical change is required to separate the water from the molecule with which it is associated.
It is also sometimes useful to measure the course of a chemical reaction where water is a reaction product. Such reactions are exemplified by carbonization of sugar, or decomposition of sodium bicarbonate into water and sodium carbonate or combustion of hydrocarbons.
It is common to analyze for the water content of a material spectrographically. A specimen to be tested has a dry carrier gas stream passed over it to evaporate the water in the specimen so that it enters the gas stream. The carrier gas stream is then analyzed spectrographically to determine the amount of water in it. This method for determining water has many problems associated with it. Among them are that water does not produce a strong spectrographic response and that the process must be performed without the possibility of water condensing or otherwise collecting in the spectrograph or in the apparatus leading to the spectrograph. It is also necessary to calibrate the spectrograph frequently to ensure that its reading accurately represents the amount of water in the gas stream passing through it. Another problem is that a spectrograph may be influenced by other materials that are similar to water which are frequently found with water. Such materials as lower alcohols, for example, will produce a spectrographic response that will influence the accuracy of a spectrograph to determine the water content of a gas stream.
The most commonly used analytic method to determine water content of a material is the Karl Fischer method which uses an alcoholic solution of iodine, sulfur dioxide and pyridine. Although the method produces accurate results the chemical basis of the method prevents its use with many materials among which are inorganic bases, carbonates, hydrogen sulfide, mercaptans and others. In addition, the reagents traditionally used in the Karl Fischer method are noxious and each material being analyzed must be dissolved in a suitable solvent that must be compatible with the Karl Fischer method of water analysis.
It is also known to analyze for water content by having the water react with calcium carbide to produce acetylene. Water reacting with calcium carbide will produce a chemically equivalent amount of acetylene and even small amounts of water react almost completely with calcium carbide. The methods known to the art that are based on acetylene production also suffer some difficulties among which are that they must be carried out with absolutely dry carrier gas and dry apparatus and that instrument calibration is extremely important so that the exact amount of acetylene produced in the carrier gas, which is the equivalent of water in the original specimen, is read.
Another problem with the known calcium carbide methods is that calcium carbide may have variable reactivity. Calcium hydroxide is one reaction product of the reaction of water and calcium carbide and at very low humidity calcium hydroxide dehydrates to calcium oxide. The observed variable activity of fresh calcium carbide may be due to the presence of some calcium hydroxide which may dehydrate in the presence of dry carrier gas, or to the presence of some calcium oxide which may hydrate in the presence of humid carrier gas. In either case the water content of the carrier gas is altered by these reactions.
One commercially available device for measuring water by conversion of water to acetylene with calcium carbide is sold under the name C-AQUA-TESTER by C. W. BRABENDER INSTRUMENTS, INC. It includes a sealed reaction chamber for the material being tested and calcium carbide, and when calcium carbide and the water in the material react to produce acetylene the water content of the material is determined by the rise in pressure in the reaction chamber. This method is slow and cumbersome because each test requires assembling and disassembling a pressure vessel, and being based on pressure, it is sensitive to temperature and the vaporization of the material being tested or the production of gas phase products by other reactions. Accordingly, it is very difficult to use to determine the water content of reactive materials, volatile liquids or solids, or of gas phase materials. The C-AQUA-TESTER is described in the article, "Determination of Moisture and Volatile Content in Coal by Pressure Change and Gas Chromatography", Jenke et al., page 843, Volume 54, No. 4, (1982), Analytical Chemistry.
Typical known methods to analyze for water are described in the article, "Determination of Traces of Water In Hydrocarbons"; page 749, Volume 34 (1962) Analytical Chemistry by Knight and Weiss, and in the article, "Trace Water Determination by Infrared Spectrometry", page 1125, Volume 34 (1962) Analytical Chemistry, by J. W. Forbes. Other literature of which the inventors are aware describe different methods and instruments for acetylene analysis but are no more descriptive of the concept for the process cr apparatus for acetylene analysis than the references cited above.